This invention generally relates to light-sensitive imaging arrays. More particularly, the present invention relates to sealing of exposed edges of organic dielectric layers to prevent undercutting of the organic dielectric layers from adversely affecting imager performance and reliability.
Photosensitive element arrays for converting incident radiant energy into an electrical signal are commonly used in imaging applications, for example, in x-ray imagers and facsimile device arrays. Hydrogenated amorphous silicon (a-Si) and alloys of a-Si are commonly used in the fabrication of photosensitive elements for such arrays due to the advantageous characteristics of a-Si and the relative ease of fabrication. In particular, photosensitive elements, such as photodiodes, can be formed from such materials in conjunction with necessary control or switching elements, such as thin film transistors (TFTs), in relatively large arrays.
X-ray imagers, for example, are formed on a substantially flat substrate, typically glass. The imager includes an array of pixels with light-sensitive imaging elements, typically photodiodes, each of which has an associated switching element, such as a TFT or one or more additional addressing diodes. In conjunction with a scintillator, x-rays are transformed into visible light for imaging with the photosensitive elements. The photosensitive elements, typically photodiodes, are connected at one surface to a switching device, typically a thin-film transistor, and at the other surface to a common electrode which contacts all the photodiodes in parallel. The array is addressed by a plurality of row and column address lines having contact pads located along the sides of the array. In operation, the voltage on the row lines, and hence the TFTs, are switched on in turn, allowing the charge on that scanned line""s photodiodes to be read out via the column address lines, which are connected to external amplifiers. The row address lines are commonly referred to as xe2x80x9cscan linesxe2x80x9d and the column address lines are referred to as xe2x80x9cdata lines.xe2x80x9d The address lines are electrically contiguous with contact fingers which extend from the active region toward the edges of the substrate, where they are in turn electrically connected to contact pads. Connection to external scan line drive and data line read out circuitry is made via the contact pads.
The common electrode, which is disposed over the top of the photodiode array provides electrical contact to the photodiode array. The photodiode array is typically overlaid with a first layer of inorganic and a second layer of organic polymer dielectric, as disclosed in U.S. Pat. No. 5,233,181, issued on Aug. 3, 1993 to Kwansnick (sic) et al. Contact vias are formed over the photodiodes in each dielectric layer to allow electrical contact to the photodiode tops by the common electrode.
Patterning of the common electrode comprises deposition, photolithography and photoresist strip, as is well known in the art. For light imagers comprising amorphous silicon, it is observed that the vias, necessary for electrical connection between the contact pads and the contact fingers, may be damaged if the photoresist is removed by a wet strip process, degrading the imager. Therefore, dry strip of the common electrode photoresist is generally used, for example, by ashing with a plasma containing O2. However, the dry strip also etches the underlying organic polymer, causing undercut of its edges under those of the common electrode. A barrier layer is typically disposed on the imager after common electrode formation, for example, see U.S. Pat. No. 5,401,668, issued Mar. 28, 1999 to Kwasnick et al., and this common electrode overhang results in poor step coverage of the barrier layer, causing degraded environmental protection and possible photodiode leakage. Thus, a need exists to address the undercutting problem.
It is desirable that the imager structure be robust both for withstanding the fabrication process and good performance in operation. As higher performance is required of imagers (e.g., noise, resolution, etc.), the necessity arises of greater patterning of the imager structure to provide the desired performance in operation and ability to withstand the rigors of fabrication and usage.
In one example of the present invention, a structure and a method of forming the structure for an imager is presented. The structure comprises an organic dielectric layer, and a common electrode, comprising a light-transmissive conductive layer, the common electrode covering the organic dielectric layer and extending beyond an exposed edge of the organic dielectric layer along a xe2x80x9cstripedxe2x80x9d segment of the common electrode.